1. Field of the Invention
The invention pertains to a hydraulic lift or elevator of the type having a hydraulic drive, a pressure accumulator, a main pump for producing a flow of hydraulic oil between the pressure accumulator and the hydraulic drive, and a changing pump for changing the pressure accumulator with hydraulic oil from a tank.
2. Description of the Related Art
A hydraulic elevator of the type indicated above is known from U.S. Pat. No. 5,349,142. In this publication, the elevator is called an energy-conservation type elevator. Speed-adjusting valves, one in the line from the pump to the drive, the other in the line from the pump to the pressure accumulator, are provided to control the acceleration/deceleration curves. It is also disclosed that the main pump is driven by an electric motor, which is controlled by an inverter.
EP-A1-829 445 describes a device in which, under certain conditions, the motor, which is coupled to the pump, acts as a generator, so that excess hydraulic energy is converted to electrical energy and can therefore be recovered.
U.S. Pat. No. 5,281,774 describes a control device for a hydraulic elevator, in which the electric motor which drives the pump is controlled by an inverter.
A hydraulic elevator with a pressure accumulator is also known from U.S. Pat. No. 5,579,868. In one of the design variants of this elevator, a first pump is installed between the hydraulic drive for the elevator and the pressure accumulator; this pump is used to influence the flow of hydraulic oil between the hydraulic drive and the pressure accumulator. A hydraulic motor is coupled to the first pump, and a controlled partial stream of hydraulic oil flows through this motor to the tank. Energy, which is exploited to operate the first pump, is recovered from the pressure difference which arises when the hydraulic oil is expanded from the hydraulic drive or pressure accumulator to the pressureless tank. A second pump is used convey hydraulic oil from the tank to the pressure accumulator, so that the pressure accumulator can be recharged again as often as necessary.
It is known from U.S. Pat. No. 6,142,259 that the pump for conveying hydraulic oil in a hydraulic elevator can be operated by an electronic power output stage. Such power output stages are known under the name “frequency converters”.